System for determining fundamental frequency of an AC signal

Abstract

A system for determining the fundamental frequency of a digitized AC signal is disclosed. Each signal zero crossing is detected, and in response to each zero crossing, two discrete impulses are generated. In response to the train of generated impulses, the system determines the fundamental frequency of the AC signal.

What is claimed is: 
     
       1. A system for determining the fundamental frequency of a digitized three-phase AC signal, in which each phase periodically crosses zero voltage at zero crossings, at which it changes from positive to negative, or vice versa, the system comprising: first means for detecting zero crossings of one phase of the AC signal;   second means for detecting zero crossings of a second phase of the AC signal;   third means for detecting zero crossings of a third phase of the AC signal;   means, responsive to said first means for detecting, for generating a plurality of discrete impulses for each detected zero crossing of the one phase of the AC signal;   means, responsive to said second means for detecting, for generating a plurality of discrete impulses for each detected zero crossing of the second phase of the AC signal;   means, responsive to said third means for detecting, for generating a plurality of discrete impulses for each detected zero crossing of the third phase of the AC signal;   means for summing said generated plurality of discrete impulses from all of said means for generating; and   means, responsive to said means for summing, for determining the fundamental frequency of the AC signal.   
     
     
       2. The fundamental frequency determination system of claim 1 in which each said means for detecting includes crossing detector means for generating a discrete non-zero value at each AC signal zero crossing. 
     
     
       3. The fundamental frequency determination system of claim 1 in which each said means for generating a plurality of discrete impulses generates two discrete non-zero impluses at each AC signal zero crossing. 
     
     
       4. The fundamental frequency determination system of claim 3 in which the sum of said two discrete non-zero impluses is a constant. 
     
     
       5. The fundamental frequency determination system of claim 4 in which each said means for detecting includes means for periodically sampling the AC signal voltage. 
     
     
       6. The fundamental frequency determination system of claim 5 in which the sampling frequency of said means for periodically sampling is much greater than the fundamental frequency of the AC signal. 
     
     
       7. The fundamental frequency determination system of claim 6 in which the fundamental frequency of the AC signal is about 60 Hz, and said sampling frequency is 1500 Hz. 
     
     
       8. The fundamental frequency determination system of claim 6 in which each said means for detecting further includes crossing detector means, responsive to said means for periodically sampling, for generating a discrete non-zero value at each periodic sample indicative of each AC signal zero crossing. 
     
     
       9. The fundamental frequency determination system of claim 8 in which each said crossing detector means includes means for generating a discrete zero value at each periodic sample not indicative of an AC signal zero crossing. 
     
     
       10. The fundamental frequency determination system of claim 1 further including notch filter means for filtering each phase of the AC signal before it reaches each said means for detecting, said notch filter means having an essentially flat response around the AC signal fundamental frequency, and essentially zero response around at least one harmonic of the AC signal fundamental frequency. 
     
     
       11. The fundamental frequency determination system of claim 10 further including anti-aliasing filter means for filtering each phase of the AC signal before said notch filter means. 
     
     
       12. The fundamental frequency determination system of claim 6 in which said constant is equal to one-half of said sampling frequency. 
     
     
       13. The fundamental frequency determination system of claim 1 in which said means for determining the fundamental frequency of the AC signal includes means for filtering said plurality of discrete impulses, said means for filtering having an essentially flat response around zero Hz, and essentially zero response around at least one harmonic of the AC signal fundamental frequency. 
     
     
       14. The fundamental frequency determination system of claim 13 in which said means for filtering said plurality of discrete impulse includes IIR averaging filter means which takes in said discrete impulses, and creates a filtered output. 
     
     
       15. The fundamental frequency determination system of claim 14 in which said means for filtering said plurality of discrete impulses further includes variable width averaging filter means which takes in the output of said IIR averaging filter means, and creates a filtered output. 
     
     
       16. The fundamental frequency determination system of claim 15 in which said variable width averaging filter means includes means for adjusting the speed of response of said means for determining the fundamental frequency of the AC signal, to changes in the fundamental frequency. 
     
     
       17. The fundamental frequency determination system of claim 15 in which said means for filtering said plurality of discrete impulses further includes FIR averaging filter means which takes in the output of said variable width averaging filter means, and creates a filtered output. 
     
     
       18. The fundamental frequency determination system of claim 1 in which said means for summing includes means for subtracting from the summed discrete impulses a signal related to a presumed fundamental frequency, so that said means for determining the fundamental frequency of the AC signal resolves the difference between the presumed fundamental frequency and the actual fundamental frequency.